Dapatkan nilai bukan nol minimum di beberapa kolom

Coba fungsi NULLIF:

SELECT LEAST(
        nullif( number1, 0 ), 
        nullif( number2, 0 ), 
        nullif( number3, 0 ), 
        nullif( number4, 0 )) 
FROM numbers

Demo --> http://www.sqlfiddle.com/#!12/641fb3 /1